Alter logic for parsing solrconfig.xml.

Not sure quite what it was, but it was failing to properly determine
if dismax was available..
Be safer when trying to load XML.

... The given file might not have loaded successfully.
Fix the logic in checking the HTTP code, and fix a strict warning.
Avoid assignments in conditions.
Merge pull request #123 from adam-vessey/7.x-solrconfig-loading

Load and parse the solrconfig.xml, leveraging XPath to do most of the lifting...